Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0224—Channel estimation using sounding signals
  • H04L25/0228—Channel estimation using sounding signals with direct estimation from sounding signals
  • H04L25/023—Channel estimation using sounding signals with direct estimation from sounding signals with extension to other symbols
  • H04L25/0232—Channel estimation using sounding signals with direct estimation from sounding signals with extension to other symbols by interpolation between sounding signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0224—Channel estimation using sounding signals
  • H04L25/0226—Channel estimation using sounding signals sounding signals per se
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/024—Channel estimation channel estimation algorithms
  • H04L25/0242—Channel estimation channel estimation algorithms using matrix methods
  • H04L25/0244—Channel estimation channel estimation algorithms using matrix methods with inversion
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/024—Channel estimation channel estimation algorithms
  • H04L25/025—Channel estimation channel estimation algorithms using least-mean-square [LMS] method
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/024—Channel estimation channel estimation algorithms
  • H04L25/0256—Channel estimation using minimum mean square error criteria
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/003—Arrangements for allocating sub-channels of the transmission path
  • H04L5/0048—Allocation of pilot signals, i.e. of signals known to the receiver
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0212—Channel estimation of impulse response
  • H04L25/0216—Channel estimation of impulse response with estimation of channel length
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/0001—Arrangements for dividing the transmission path
  • H04L5/0003—Two-dimensional division
  • H04L5/0005—Time-frequency
  • H04L5/0007—Time-frequency the frequencies being orthogonal, e.g. OFDM(A), DMT

Definitions

• the invention relates to the field of wireless communication and more particularly to a process for estimating the channel based on the PSS signal in a LTE communication network, and a receiver for doing the same.
• Digital wireless communications are being widely used throughout the world particularly with the latest development of the Orthogonal Frequency Division Multiplex (OFDM systems) and the last evolution, namely the so-called Long Term Evolution (LTE) systems.
• OFDM Orthogonal Frequency Division Multiplex
• LTE Long Term Evolution
• a User Equipment When a User Equipment (UE) wishes to access an LTE network, it must initiate a cell search procedure consisting of a series of synchronization steps by which the UE determines time and frequency parameters particularly necessary for the purpose of demodulating the downlink and also for getting critical system parameters.
• UE User Equipment
• the cell search procedure is based on the use of two particular synchronization signals being broadcast in each cell, namely the so-called Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS).
• PSS and SSS are sequences of length 62 which are mapped to the central 62 sub-carriers (not including the d.c.) independently of the transmission bandwidth, as illustrated in Figure 1.
• LTE 1 the largest unit of time is the 10ms radio frame, which is subdivided into ten 1ms subframes, each of which being split into two 0.5 ms slots.
• Each slot comprises six to seven OFDM symbols in accordance with the length of the cyclic prefix.
• resources are grouped in units of 12 subcarriers and each block of 12 subcarriers, during one slot, is called a Resource Block (RB), the latter being divided into Resources Elements (RE) which lasts for one OFDM symbol.
• RB Resource Block
• RE Resources Elements
• FIG. 2 recalls the general structure of the LTE sub-frame comprising both the broadcasted PSS and SSS signals.
• the UE in order to synchronize, the UE has first to detect the PSS, which detection is then used for the purpose of the decoding of the SSS which provides the identification of the cell, such subsequent extraction being further required for the purpose of the decoding of the pilots or Reference Signals necessary for getting critical system parameters and also for achieving an accurate estimation of the channel.
• Such channel estimation is highly desirable since it significantly improves the efficiency of the subsequent synchronization phases;
• the knowledge of the channel allows the designer the possibility to consider coherent SSS detection methods which are known to be more efficient.
• PSS Primary Synchronization Channel
• MMSE Minimum Mean Square Error
• LTE Long Term Evolution
• LS Least Square
• the d.c. sub-carrier is interpolated by averaging the two neighboring sub-carriers..
• the missing subcarrier -32 is interpolated by means of subcarrier # -31.
• the method involves the steps:
• F 1 is the 64 x L matrix, obtained by selecting the first L columns of the Nx N DFT or FFT matrix, ⁇ 2 is the noise variance and C 4 is the channel time domain impulse response covariance matrix.
• step 50 Perform a Least square (LS) channel estimation on the 62 sub-carriers containing the PSS (step 50) - reconstruct the LS channel estimation on the 64 central sub-carriers by interpolating on the two missing sub-carriers (step 51);
• LS Least square
• the method involves the steps of:
• step 60 Perform a Least square (LS) channel estimation on the 62 sub-carriers containing the PSS (step 60) - reconstruct the LS channel estimation on the 64 central sub-carriers by interpolating on the two missing sub-carriers (step 61 );
• LS Least square
• step 66 Perform a DFT or FFT of the result (step 66).
• the method may involve the steps of : - Perform a Least square (LS) channel estimation on the 62 sub-carriers containing the PSS (step 70)
• LS least square
• step 72 Applying an IFFT to the interpolated channel estimation (step 72); - Extract the first samples corresponding to the Cyclic Prefix (CP) length (step 74).
• step 75 Perform a DFT or an FFT of the result (step 75).
• the method involves the steps of :
• step 81 - reconstruct the LS channel estimation on the 64 central sub-carriers by s interpolating on the two missing sub-carriers (step 81);
• the method involves the steps of:
• the invention also provides a Receiver for a LTE digital communication network comprising means for extracting a Primary Synchronization Signal (PSS) from a received signal and further involving means for spanning said PSS on the 64 sub- carriers so as to facilitate the channel estimation.
• PSS Primary Synchronization Signal
• Figure 1 illustrates the frequency allocation for synchronization signal in LTE.
• Figure 2 illustrates he general structure of the LTE sub-frame comprising both the broadcasted PSS and SSS signals.
• Figure 3 illustrates the PSS mapping to sub-carriers.
• Figure 4 illustrates a first embodiment of the channel estimating method.
• Figures 5-10 shows alternate embodiments corresponding to some situations where the knowledge of C h , ⁇ 2 or L is presumed to be not known.
• the OFDM modulation is widely spread in many existing standards (such as DAB, DVB-T, WiMAX, IEEE 802.16, ADSL, WLAN IEEE 802.11a/g, etc..) and expected to be used in many future developing standards.
• the OFDM converts a broadband frequency-selective channel into a multiplicity of parallel narrow-band single channels. This is achieved by means of the insertion of a guard interval (so-called
• Cyclic Prefix CP between the individual symbols.
• Such guard interval is assumed to be temporally long enough to compensate for jitter, i.e. the variation of the delays experienced by the transmitted OFDM symbols through the channel. This prevents the appearance of inter-symbol interference (ISI).
• ISI inter-symbol interference
• the PSS and SSS signals are sequences of length 62 mapped to the central 62 sub-carriers (not including the d.c.) independently of the transmission bandwidth, and which are generated from frequency-domain Zadoff-Chu sequence according to the following formula :
• M is the ZC sequence root.
• Three different PSS identities i.e. ZC sequence roots
• the Primary Synchronization Signals sequence mapping to sub- carriers is particularly illustrated in figure 3.
• the transmitted PSS sequence can be detected by performing a correlation of the received signal against the three possible PSS sequences and by detecting the correlation peak.
• the detection of the sequence identity and time location allows the OFDM symbol synchronization and the cell ID decoding.
• the PSS can be used as a pilot sequence allowing channel estimation that can be used for the SSS detection. This is made possible by the fact that the SSS and PSS are mapped both to the (same) central sub-carriers, and that the OFDM symbols containing the PSS and the SSS are consecutive as shown in figure 2.
• Y be the vector of length 62 of the received signal in the 62 sub-carriers corresponding to the PSS.
• the LS (Least Square) channel estimator at the PSS positions is given by the following vector :
• d[k] is the transmitted PSS symbol at position k .
• the MMSE estimator on the 64 central sub- carriers can be expressed as
• H MMSE F L (F?F P + ⁇ 2 C- 1 Y F? HLS
• F p is the 62 x L matrix, obtained by selecting the rows corresponding to the PSS positions and the first L columns of the 64x 64 Discrete Fourier Transform (DFT) matrix.
• F L is the matrix which is obtained by selecting the first L columns of the 64x64 DFT matrix, and
• the MMSE estimator shows a significant complexity due to the two following grounds:
• the method involves LS channel estimation on the 62 sub-
• HLS denotes the estimated vector
• the method involves an interpolation based on reconstructing the LS channel estimation in the two missing sub-carriers (namely sub-carriers #-32
• HLS shows to be spanning the 64 central sub-carriers, which is a condition for a significant reduction in the complexity of the MMSE (Minimum Mean Square Error) estimation.
• MMSE Minimum Mean Square Error
• step 41 for the purpose of performing such reconstruction.
• this can be achieved by any appropriate function (for example a linear combination) of the channel estimation in the adjacent sub-carriers.
• the channel estimation in the d.c. sub-carrier is made by averaging the two neighboring sub-carriers (i.e. neighbor in the left and neighbor in the right with respect to the central d.c. position).
• the channel estimation in the sub-carrier at the edge is set equal to the channel estimation in the adjacent sub-carrier (i.e. sub-carrier -31 ).
• the method involves the following steps:
• step 42 apply an IFFT; step 43. extract the L first samples, L corresponding to the length of the channel); step 44. multiply by [F L F L + ⁇ C h step 45: Zero-pad the resulting signal to length 64. step 46 Apply an FFT
• step 44 - which requires the inversion of a matrix - is very simple to carry out since the matrix shows to be diagonal and, furthermore, of a size LxL.
• the interpolation step 41 shows to be very advantageous since it allows to significantly reduce the level of the complexity of the MMSE channel estimation.
• Figures 5-10 shows different embodiments corresponding to some situations where the knowledge of C h , ⁇ 2 or L is presumed to be not known. In such case, the following approximations can be made.
• the method comprises the following steps :
• Step 50 ⁇ Perform LS channel estimation on the 62 sub-carriers containing the PSS (Step 50 ⁇ - reconstruct the LS channel estimation on the 64 central subcarriers by interpolating on the two missing sub-carriers (step 51);
• the method achieves the channel estimation as follows :
• Step 60 Perform LS channel estimation on the 62 sub-carriers containing the PSS (Step 60) - reconstruct the LS channel estimation on the 64 central subcarriers by interpolating on the two missing sub-carriers (step 61 );
• Step 71 Perform LS channel estimation on the 62 sub-carriers containing the PSS (Step IQl - reconstruct the LS channel estimation on the 64 central subcarriers by interpolating on the two missing sub-carriers (step 71);
• step 72 Applying an IFFT to the interpolated channel estimation (step 72); - Extract the first samples corresponding to the Cyclic Prefix (CP) length (step 73);
• step 81 reconstruct the LS channel estimation on the 64 central sub-carriers by interpolating on the two missing sub-carriers
• step 82 Applying an IFFT to the interpolated channel estimation (step 82) ; - Keep the samples corresponding to the existence of non-zero taps in C h and put zeros elsewhere (step 83) ;
• step 91 reconstruct the LS channel estimation on the 64 central sub-carriers by interpolating on the two missing sub-carriers
• Figure 10 illustrates a seventh embodiment of a channel estimation involving the steps of :
• step 101 - reconstruct the LS channel estimation on the 64 central sub-carriers by interpolating on the two missing sub-carriers (step 101 );
• step 104 Keep only samples with power exceeding a threshold defined w.r.t the maximum power (may also be limited to only the samples in the CP)) (step 104).
• step 105 Perform an FFT of the result (step 105).
• Such method clearly allows an effective channel estimation at the very early stage of the synchronization procedure, namely during the extraction of the PSS, without requiring great amount of digital resources.
• Such channel estimation can then be used for subsequent operations, e.g. for the detection of the SSS.

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• Power Engineering (AREA)
• Physics & Mathematics (AREA)
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• 2009
  • 2009-06-19 EP EP09368020A patent/EP2264961A1/en not_active Withdrawn
• 2010
  • 2010-06-18 CN CN2010800274957A patent/CN102461102A/zh active Pending
  • 2010-06-18 US US13/379,155 patent/US8804863B2/en not_active Expired - Fee Related
  • 2010-06-18 JP JP2012515397A patent/JP5661753B2/ja not_active Expired - Fee Related
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